The present invention relates to a process for producing a diaphragm for an acoustic device of fully carbonaceous materials. More particularly, the invention relates to a process for producing an acoustic carbon diaphragm having a high hardness, a high strength and a high elasticity as compared to a diaphragm manufactured from a conventional diaphragm material. The diaphragm may be used as a speaker and a microphone. It exhibits less deformation by an external force due to excellent rigidity, as well as low sound distortion, a wide sound reproduction range, and distinct sound quality. In addition, the diaphragm imparts a high rigidity to an entire vibration system owing to an integrated structure of the diaphragm and a voice coil bobbin, which eliminates energy loss during transmission of a driving force generated in a voice coil to the diaphragm, thereby providing an excellent responsiveness to an input signal. The diaphragm is suitable for digital audio applications.
A diaphragm intended for a speaker and a voice coil bobbin should possess the following properties:
(1) low density,
(2) high Young's modulus,
(3) high sound propagating velocity,
(4) adequately large internal loss of vibration,
(5) stability against variation in the atmospheric conditions, no deformation nor change of properties, and
(6) suitability for a simple and inexpensive manufacturing process.
More specifically, the material for the diaphragm is required to have a wide sound reproduction range in high-fidelity over a broad frequency band. To efficiently and distinctly produce sound quality, the material should have high rigidity, with no distortion such as creep against external stress. To further increase the sound velocity from the equation of EQU V=(E/p).sup.1/2
where V: sound velocity, E: Young's modulus, p: density, the material is required to have small density and high Young's modulus.
In addition to the above-mentioned conditions, in the case of a voice coil bobbin, the material should be resistant to the Joule heat generated by a voice current flowing in a voice coil.
The conventional materials for the diaphragm and voice coil bobbin include paper (pulp), plastic, aluminum, titanium, magnesium, beryllium, boron as basic materials, and further contain glass fiber, carbon fiber compositely mixed with the basic material, or processed to metal alloy, metal nitride, metal carbide, or metal boride. However, the paper, plastic and their composite materials have a small Young's modulus and small density. Thus, the sound velocities of these materials are low. Vibration division occurs in a specific mode and the frequency characteristics in the high frequency band of the materials are particularly low, resulting in difficulty in producing distinct sound quality. In addition, these materials are feasibly affected by their external environment such as temperature and moisture, causing deterioration in the quality and ageing fatigue, thereby disadvantageously decreasing the characteristics.
Plates of aluminum, magnesium, or titanium have also been employed. The sound velocities of these materials are high, but the materials have sharp resonance phenomenon in high frequency band with small internal loss of vibration, or ageing fatigue such as creep occurs in the materials, thereby disadvantageously deteriorating their characteristics.
Boron, beryllium, and their nitrides, carbides and borides provide excellent physical properties. Tweeters which use these materials in their diaphragms possess sound reproduction limits in the audible frequency bands or higher, thereby correctly producing natural sound quality without transient phenomenon by the signals in the audible band. However, these materials are very expensive, and are difficult to machine. In particular, the conventional process for producing a diaphragm by rolling and press molding is not practical and a depositing method such as C.V.D. or P.V.D. should be employed. These processes are expensive and it is difficult to produce speakers of large size.
Because the conventional material for the voice coil bobbin is typically paper (pulp), such as kraft paper, the rigidity of the entire vibration system decreases even if materials having excellent physical properties are used for a diaphragm. The rigidity of the entire vibration system also decreases due to the presence of a bonding layer for bonding the diaphragm to the voice coil bobbin, and an energy loss occurs at the bonding layer when transmitting the driving force generated in the voice coil to the diaphragm.